In the prior art it is known to obtain an indication of color and brightness characteristics of a paper web during manufacture by an on-line measurement of reflectance value (Rg), but this measurement is decidedly different from that necessary for actual color and brightness characterizations. Accordingly, such a measurement must be accompanied by very frequent off-line testing, so as to enable an adequate empirical calibration of the measuring instrument. Further, a separate set of calibration parameters is required for each grade and weight of paper. Off-line instruments which adequately measure these characteristics require that a pad of several thicknesses of paper be exposed to the light source aperture so that a different reflectance value (R.infin.) is obtained. Obviously this is impossible with an on-line instrument unless the far more inaccessible reel itself is tested.
Only where the on-line measured reflectance value (Rg) approaches the off-line value (R.infin.), as in instances of paper of extremely high opacity such as heavily coated or heavily dyed paper, can the above problems be minimized to the point where accuracy becomes sufficient for control purposes.